As is known, chemical, physical and biological samples can be identified and analyzed by means of Raman spectroscopy. When a sample is excited with light, e.g. monochromatic laser light, a small part of the light is inelastically scattered in addition to the absorption and emission. The resulting signals characteristic for each sample are called Raman signals. They are spectrally shifted toward the excitation wavelength both to long wavelengths (Stokes range) and to shorter wavelengths (anti-Stokes range). Since, at room temperature, the intensity of the Raman signals is typically higher in the Stokes range, the Stokes range is preferably used for the identification and analysis of a sample.
Typically, as the excitation wavelength in Raman spectroscopy, laser light is used from the ultraviolet (UV) spectral range, e.g. 260 nm, up to the near infrared (NIR) spectral range, e.g. 1500 nm. The selection of the excitation wavelength is made according to the sample characteristic or the type of the application.
The cross-section for the generation of the Raman signals depends on the excitation wavelength λ, proportionally to λ−4. An excitation at shorter wavelengths can therefore result in higher Raman intensities.
For special samples, excitations, especially in the ultraviolet and visual (VIS) spectral range, may result in resonance effects (Resonance Raman spectroscopy) and amplify the weak Raman signals by several orders of magnitude.
Nevertheless, fluorescence signals may be generated in addition to the Raman signals. Typically, they mask the weak Raman signals and thus make a Raman spectroscopic analysis of the sample impossible or difficult. If this characteristic of the sample is known prior to the examinations, the proper selection of the excitation wavelength, e.g. in the NIR spectral range, permits to avoid the simultaneous excitation of fluorescence signals.
Thus, the selection of the excitation wavelength determines the quality of the measured Raman spectra. Thus, particularly in the case of unknown samples, preliminary examinations are required for a proper selection of the excitation wavelength, which is then predetermined to a large extent for the optical measurement system.
The optical measurement system or the transfer optical system, i.e., the elements for exciting samples and for collecting the Raman signals of the samples for the purpose of spectral analysis are also called optodes.
A suitable optode guides the excitation light to the sample. A band-pass filter specially adapted to the excitation wavelength suppresses the interfering light emitted from the excitation source, such as for example plasma lines in gas lasers or amplified spontaneous emission (ASE) in diode lasers. Even unwanted signals from silica fibers with which the excitation light has been transported to the optode may be filtered out in this manner. These band-pass filters have only a narrow transmission curve and are selected to the excitation wavelength.
The light scattered back from the sample is collected in the collection path and imaged into a system for spectrally selective detection by a suitable optical system. For this purpose, dispersive elements for spectral separation and multichannel detectors, e.g. CCDs, are typically used.
There are several orders of magnitude between the intensity of an excitation laser and the intensity of the generated Raman signals. Overdriving or saturation of pixels of a CCD has to be avoided here because excess charges are transferred into adjacent pixels (“blooming”) and may lead to artifacts in the Raman spectra. Consequently, the Raman signals have to be spectrally separated from the elastically scattered laser light in the collection path before reaching the detector. This typically takes place in spectrally selective elements, such as for example monochromators or optical filters. Long-pass filters or notch filters can be chosen for the detection of Stokes lines. These are adapted to the wavelength of the excitation light source and the spectral inspection range as well as the band-pass filter located in the excitation path.
Further optical elements, such as lenses, mirrors, filters and glass fibers, are located within the optode and are typically surrounded by a housing for protection. The laser light and the Raman signals get to the sample or back into the optode by passing through an optical window.
The examination of unknown substances, which is of interest e.g. for security-related applications in airports and border checkpoints, for medical or police-related applications, or in the private domain, is considerably complicated by the unique predefinition of the light sources, optical systems and filter sets. This is especially true for the portable devices for on-site measurements that are to be preferred for the above-mentioned applications.
In conventional arrangements of optical components for the Raman spectroscopy, the optical elements are fixedly mounted within an optode. Due to their spectral characteristics, however, the filters (band-pass filter and e.g. long-pass filter) have to be replaced for a change of the excitation wavelength. This requires a reconstruction of the optode.
Since the optical elements within an optode are adjusted and mounted to fixed positions, the distance between an excitation lens or collecting lens and the optical window of an optode may not be varied in a measurement system. This distance determines the position of the laser focus (or the collection spot) at the outside of the optode. In the case of a fixed distance, these positions cannot be adjusted to the optical characteristics of e.g. a transparent or turbid sample. Again, an expensive reconstruction of the optode is required.
In “Multi-excitation Raman spectroscopy technique for fluorescence rejection” OPTICS EXPRESS, vol. 16 no. 15, 21 Jul. 2008, McCain et al. describe a laser system having eight adjacent (782.6 nm-794.3 nm) wavelength stabilized diode lasers. Although eight different excitation wavelengths are used in this case, they are to be regarded as comparable with respect to the above-mentioned points, particularly regarding the characteristics of the Raman scattering. These wavelengths do not serve the selection of an excitation that is best for the Raman effect, but the selection of the Raman signals against the interfering fluorescence by measuring the spectra upon excitation with different wavelengths. In this method, known as “Shifted Excitation Difference Raman Spectroscopy” (SERDS), spectra are measured at only slightly different excitation wavelengths. Since, in the Raman spectra of both excitations, the Raman signals are shifted by the excitation shift, but comprise background signals of the same kind (e.g. by fluorescence), the Raman signals of the sample can be separated from the background signals (e.g. fluorescence signals) by differentiation between the spectra.
A band-pass filter was not commercially available for the range of the eight lasers. Therefore, a short-pass filter has been used to suppress the ASE in the Stokes range. A long-pass filter (F2 in FIG. 3) blocks the laser light. In this case, too, a change to an excitation wavelength which could lead to a significantly changed Raman intensity, e.g. 488 nm, would involve an adaptation of the band-pass filter and the long-pass filter and thus an expensive reconstruction of the described measurement system.
The U.S. Pat. No. 7,982,869 also addresses the application of SERDS. It describes a Raman analysis apparatus which is able to detect the presence of selected substances of interest by combining three-dimensional Bragg elements with conventional lasers and detectors, wherein the apparatus is inexpensive and manufactured in the size of few cubic meters, so that it may be used as a portable device. Particularly, a more complex Raman analysis can be effectuated by means of laser sources for two closely adjacent excitation wavelengths because the fluorescence background is independent from the excitation wavelength, whereas the spectral lines shift with the change of the excitation wavelength. In this case, the result of the Raman scattering can be collected by the same optical system, when both laser sources excite the sample sequentially. Further disclosed is a subtraction of the two excitation spectra in order to eliminate the fluorescence background, and a circuit for the analysis of the difference spectrum. However, a control, optimization or variation of the excitation wavelength(s) for the Raman signal is not disclosed.
The adjustment of the position of the excitation spot is described in the patent application US 2004/0160601 for a Raman spectroscopy arrangement having a low spectral resolution for use in portable and/or handy analysis devices. Here, the focus of the excitation beam can be adjusted by replacing an optical and mechanical component (“end cap”). But this adjustment can only be effectuated in predetermined discrete steps.